The present invention relates to optoceramics, their use and methods for their manufacture. The present invention further relates to refractive, diffractive or transmittive optical elements made of optoceramics as well as imaging optics comprising such optical elements.
For the purposes of the present invention, the term “optoceramic” refers to an essentially single-phase polycrystalline oxide-based material having a high transparency. Optoceramics are accordingly understood to be a specific subgroup of ceramics.
For the purposes of the present invention, the term “single-phase” means that more than 95% of the material, preferably at least 97%, more preferably at least 99% and particularly preferably from 99.5 to 99.9%, of the material is present in the form of crystals having the target composition. The individual crystallites are closely packed and densities of at least 99%, preferably 99.9% and more preferably 99.99%, based on the theoretical values are achieved. The optoceramics are thus virtually pore-free.
Optoceramics differ from conventional glass-ceramics in that the latter comprise not only a crystalline phase but also a high proportion of an amorphous glass phase. Furthermore, conventional ceramics do not have the high densities present in optoceramics. Neither glass-ceramics nor conventional ceramics can have the advantageous properties of optoceramics, such as particular refractive indices, Abbe numbers, values for relative partial dispersion and especially the advantageous high transparency for light in the visible range and/or in the infrared range.
A main objective in the development of imaging optics is to achieve a satisfactory optical quality at a compact and very light structure of the optics. Particularly for applications in digital image recording in electronic appliances, such as for example in lenses of digital cameras or in cameras built into mobile telephones, etc, the imaging optics have to be very small and light. In other words, the total number of imaging lens components should be kept to a minimum. This requires transparent materials having a high refractive index and/or very low dispersion so as to make it possible to design very compact imaging optics with approximately apochromatic imaging behaviour. In microscopy, virtually diffraction-limited imaging optics are required, both for ocular lenses and for objective lenses.
For night vision instruments, IR lenses and IR spectral systems, transparent optics which have a high transmission both in the visible spectral range (from 380 to 800 nm) and in the near IR to far infrared spectral range up to 7000 nm, preferably up to 10 000 nm, are required. In addition, these optics have to be particularly resistant to external influences such as mechanical stresses, shocks, temperature changes, pressure changes and if appropriate aggressive chemicals.
Materials as described above can also be employed in other technologies, such as for example digital projection or display techniques in general and also in predominantly monochromatic applications such as optical storage technologies, in which, for example, compact systems are achieved with the aid of materials having a high refractive index.
Owing to their stability to high temperatures as well as aggressive chemicals, optoceramics are also suitable for producing windows in high-temperature applications, e.g. inspection windows for high-temperature furnaces, and also as encapsulation material for alkali metal vapour lamps.
At present, the development of imaging optics is limited by the optical parameters of the available materials. The available glass melting and glass forming techniques make it possible to produce only those types of high-quality glass which, in an Abbe diagram in which the refractive index is plotted against the Abbe number, are below a line which runs approximately through the points Abbe number=80/refractive index=1.7 and Abbe number=10/refractive index=2.0. Such an Abbe diagram is shown by way of example in FIG. 1, where the above-described imaginary line is denoted by a broken line. More precisely, glasses having a refractive index in the range from about 1.9 to about 2.2 and an Abbe number in the range from 30 to 40 tend to be unstable, which makes it very difficult to produce such glasses in relatively large quantities and satisfactory quality. Likewise, glasses having a refractive index in the range from about 1.8 to about 2.1 and an Abbe number in the range from about 35 to 55 tend to be unstable.
The definitions of refractive index (refractive index at a wavelength of 587.6 nm) nD, Abbe number νd and relative partial dispersion (for example Pg,F) are, in principle, known per se to those skilled in the art. More precise descriptions of these terms can be found in the technical literature. For the purposes of the present invention, the terms are used in accordance with the definitions in “The properties of optical glass”; Bach, Hans; Neuroth, Norbert (Editors), Berlin (inter alia): Springer, 1995; or Schott, “Series on glass and glass ceramics”, science, technology and applications; 1, XVII, page 410, 2nd, corr. print., 1998, XVII, page 414.
For the present purposes, the transparency to visible light or to infrared radiation is the pure transmission.
Apart from the requirements in terms of transparency, refractive index and Abbe number, the relative partial dispersion plays a large role in the choice of an optical medium. To produce virtually apochromatic optics, a combination of materials having virtually identical relative partial dispersion but a large difference in the Abbe number is necessary. When the partial dispersion Pg,F is plotted against the Abbe number, most glasses lie on a line (“normal line”). Such a diagram is shown by way of example in FIG. 2. To produce apochromatic optics, materials whose combination of Abbe number and relative partial dispersion deviates from this behaviour are thus desirable.
Materials which in the Abbe diagram shown in FIG. 1, are above the abovementioned imaginary line are at present exclusively single crystals or polycrystalline materials.
However, the production of single crystals by the known crystal drawing processes is very costly and subject to considerable restrictions in respect of the chemical composition. Furthermore, single crystals cannot be produced with a shape close to the end shape for most applications, resulting in a considerable outlay for final machining, possibly in combination with a high removal of material. This also means that it is frequently necessary to produce single crystals which are significantly larger than the optical element desired in the end.
Although polycrystalline ceramics can be produced over a broader composition range, they frequently have unsatisfactory optical qualities, in particular as far as the homogeneity of the refractive index and the transparency are concerned. Only few composition ranges and structure types in which transparent ceramics having a satisfactory optical quality can be produced have been known hitherto.
Polycrystalline ceramics have therefore been used only to a limited extent in optical applications up to now.
The Japanese published specification JP 2000-203933 discloses, for example, the production of polycrystalline YAG by means of a specific sintering process. Furthermore, the production of polycrystalline YAG of optical quality, for example for doping with laser-active ions such as Nd, has recently also been successful.
U.S. Pat. No. 6,908,872 describes a translucent ceramic which must contain barium oxide as a necessary constituent of the ceramic. The ceramics have a perovskite structure and are paraelectric. However, ceramics which contain such barium-containing phases having a perovskite structure often have an unsatisfactory optical imaging quality. This results from the tendency of many perovskites to form distorted ferroelectric crystal structures and thus lose their optical isotropy. This leads, inter alia, to undesirable double refraction of the crystals of which the ceramic is made up. Furthermore, it has been found that the transmission of the ceramics in the region of blue light (around 380 nm) is unsatisfactory.
Ji et al. (“La2Hf2O7: Ti4+ Ceramic scintillator for X-ray imaging”, J. Mater Res. Vol. 20 (3) 567-570 (2005)) describe transparent ceramics having the composition La2Hf2O7. The material described there is doped with titanium. Further ceramics of this type which are doped with other dopants such as Eu4+, Tb3+ or Ce3+ are described, for example, in Ji et al. (“Preparation and spectroscopic properties of La2Hf2O7 Tb” Materials Letters, 59 (8-9), 868-871, April 2005 and “Fabrication and spectroscopic investigation of La2Hf2O7-based phosphors”. High Performance Ceramics III, parts 1 and 2, 280-283; 577-579 1:2). Furthermore, the abovementioned authors have also described undoped variants of the abovementioned compounds (“Fabrication of transparent La2Hf2O7 ceramics from combustion synthesized powders” Mat. Res. Bull. 40 (3) 553-559 (2005)).
None of the abovementioned publications describes the use of the materials as lens material; only the use as detector material for CT instruments is reported. Although the optical quality displayed by the compounds may be satisfactory for the desired use as detector material for CT instruments, it is unsatisfactory for precise imaging of an object by means of an optical system.
Furthermore, EP 1 992 599 A1, in the name of the present applicant, discloses optical elements and imaging optics based on pyrochlore ceramics of various compositions.
More recent developments in patents in the field of optically transparent inorganic ceramic materials are summarized, for example, in the review article by Silva et al. (Recent Patents on Material Science 2008, 1, 56-73). This article describes optically transparent inorganic materials which include aluminium oxides, aluminium oxynitrides, perovskites, yttrium aluminium garnets, PLZT ceramics, Mg—Al spinels, yttrium oxides and REE oxides.
To solve the abovementioned problems, spinel ceramics having the composition MgO—Al2O3 have also been taken into consideration for some time. Examples of such ceramics are disclosed, for example, in the following documents, namely U.S. Pat. No. 3,516,839, U.S. Pat. No. 3,531,308, U.S. Pat. No. 4,584,151, EP 0 334 760 B1, U.S. Pat. No. 3,974,249, WO 2006/104540 A2, U.S. Pat. No. 3,767,745, EP 0 447 390 B1, U.S. Pat. No. 5,082,739, EP 0 332 393 A1, U.S. Pat. No. 4,273,587, GB 2,031,339, JP 04016552 and WO 2008/090909. However, the refractive indices of the transparent ceramics based on Mg—Al spinel established at present are in the region of nD=about 1.72 and are not high enough to be able, firstly, to meet the requirements of new fields of application (nD>1.72) and, secondly, to be able to compete with existing glass solutions.